custom loader for <amp-apester-media>

[mrsufgi]

Hey All,

This PR exists only because I did some bad merge/rebases,
In short, this feature add custom loader into elements. (based on createPlaceholderCallback() )

hopefully, this could be merged ASAP.

@dvoytenko @OmriKeret

Best,
Ori

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If you signed the CLA as a corporation, please let us know the company's name.

[mrsufgi]

I signed it!

[mrsufgi]

this replace #6053.

[dvoytenko]

@mrsufgi Merged! Thanks a lot!